Albert Einstein acquired Swiss nationality in 1901.
Einstein was born in Ulm in the German Empire, but he later pursued Swiss citizenship after moving to Switzerland. He became stateless in 1896 after renouncing his Württemberg citizenship, partly to avoid compulsory military service. He then studied at the Swiss Federal Polytechnic in Zurich and built important personal and professional ties in the country.
In 1901, Switzerland granted him citizenship. This status was significant during his years at the Patent Office in Bern, where he worked while developing ideas that led to his famous 1905 papers. He retained Swiss citizenship for the rest of his life, even when he later held other national affiliations.
The answer is sometimes confused with German or American nationality. Einstein was also connected with Germany during his career, and he became a United States citizen in 1940 after settling in Princeton. However, the nationality he acquired in 1901 was Swiss. Switzerland also became the setting for his education, early career, and Annus Mirabilis work.